Cellulose acylate films having low degrees of acylation such as diacetyl cellulose (DAC) films having a low degree of acetylation readily exhibit retardation, and can act as optical compensation films without addition of a retardation enhancer.
Poly(vinyl alcohol) adhesives have been typically used in bonding of polarizers to optical compensation films comprising cellulose acylate. In this method, the surfaces of the cellulose acylate films should be preliminarily hydrophilicized with saponifying solutions or the like. Unfortunately, the cellulose acylate films having low degrees of acylation are partially dissolved into the saponifying solutions, causing process contamination, during a saponifying step. Accordingly, an adhesive which requires no saponification has been desired.
Photocurable adhesives have received attention as adhesives used in bonding of polarizers to protective films for a polarizing plate (for example, see Patent Documents 1 and 2). These photocurable adhesives can bond the polarizers to the films while the films undergo no saponification, and can be expected to be used in bonding of the cellulose acylate films having low degrees of acylation.
Unfortunately, the photocurable adhesives have raised new problems.
In general, it is known that an adhesive permeates into an object such as films to some extent to form a mixed layer having an enhanced adhesive force. On the other hand, it has been found that if more than a predetermined permeation rate of the adhesive permeates into the object, the amount of the adhesive present at the interface will reduce, causing poor adhesion conversely.
While a thick coating of a photocurable adhesive can avoid this poor adhesion, this method increases the cost of the adhesive. In particular, this method applied to thin films (40 μm or less) causes ready curling of bonded films due to contraction of the permeated photocurable adhesive and a reduction in polarization of the bonded films due to a deviation in orientation angle in the films caused by the heat generated during photocuring of the permeated adhesive. To avoid these problems, a thin coating (for example, a thickness of 1 μm or less) of the photocurable adhesive is required. Unfortunately, such a thin coating of the adhesive readily causes poor adhesion. Accordingly, it is essential to control the permeation rate of the photocurable adhesive for bonding of the thin films in particular. This requirement forces, for example, blend of a photocurable adhesive less permeable into films so that the selection of adhesives is narrowed.
An enhancement in the adhesiveness has been attempted by control of the difference in the solubility parameter (SP) value between a photocurable adhesive and a film (for example, see Patent Document 3). Unfortunately, this method cannot attain sufficient adhesiveness of thin films, resulting in a ready reduction in polarization due to a deviation in the orientation angle from the optical axis of in the film.
Consequently, prevention of poor adhesion, curling of polarizing plates, and a reduction in polarization of the polarizing plates caused by bonding of thin optical compensation films with photocurable adhesives have been waited.